#include<iostream>
#include<cmath>
using namespace std;
int main(){
	double f=0,n=0;
	double x1=(1+sqrt(5.0))/2,x2=(1-sqrt(5.0))/2,x3=sqrt(5.0);
	cin>>n;
	f=(pow(x1,n)-pow(x2,n))/x3;
	printf("%.2f",f);
	return 0;
}
